GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 351-4
Presentation Time: 9:00 AM-6:30 PM


MATOS, Javier, 159 North 700 West, Provo, UT 84601,

The study area is in the central segment of Bolivian Eastern Cordillera, located in Cochabamba. The ultramafic/mafic, intermediate alkaline and carbonatite rocks, crop out as dykes, sills, and stocks, this last one of subvolcanic breccia composition from Permian to Cretaceous. The intraplate tectonic setting subjected to the lithospheric extension regime, culminates into an aborted rift is the relevant feature of the area.

The igneous rocks intruded to the meta-sedimentary rocks of Ordovician that consists of quartzites and slates. The breccias crop out in two hills and other bodies aligned from N to S. The subvolcanic bodies intruided during three pulses. The first of them intruded the breccia, while a posterior pulse formed the dikes and sills and finally, another intrusion of dykes/sills cuts to the dykes and breccias coming from the previous pulses.

Petrographic analyses define that the dykes and sills consists of basanite, tephrite and phonolitic tephrite. These rocks are composed of olivine, augite, diopside, lamprobolite, nepheline and plagioclase. The mineral characteristic of basanite is olivine that forms reaction rim with pyroxene and lamprobolite. The metasomatic minerals are calcite and siderite invading the matrix and modifying the composition of the rock associated to chlorite and phlogopite, and cancrinite in replacement of nepheline.

The alkaline magmatism is defined by chemical analysis with LREE enrichment by metasomatism, and a decrease in the content of HREE. They derived from spinel and granatiferous mantle. Additional to this, the rocks are classified as intraplate magmatism, in agreement with the extensional event occurred during the Permian to Cretaceous, exposed in the axis of the Eastern Cordillera.